Automatic control system



July 7, 1936. P Q FARNHAM 2,047,166

AUTOMATIC CONTROL SYSTEM Filed Jan. 15, 1952 Patented July 7, 1936 UNITED STATES 2,047,166 AUTOMATIC CONTROL SYSTEM Paul 0. Farnham, Boonton N. J., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application January 15, 1932, Serial No. 586,936 29 Claims. (01. 250'-2o) This invention relates to circuit arrangements for automatically controlling the amplification of a radio amplifier to maintain a substantially constant output level as the strength of the im- 5 l press-ed radio voltage varies over wide limits.

In the reception of modulated carrier waves, .it is known that the audio output level may be rendered substantially independent of fluctuations in the strength of the received signal energy 1 by employing, as a control grid bias voltage on a carrier and/or intermediate frequency amplifier, the direct current component obtained by rectification of the amplified radio energy. Separate rectifiers have been employed for the demodulation and for the rectification of the amplified signal energy and, according to another system, a single tube has served both as the demodulator and the rectifier. A gain control system employing a single tube as the demodulator and as the rectifier of an automatic gain control system is described and claimed in my copending application, Ser. No. 524,383, filed March 21, 1931.

An object of the present invention is to provide an automatic control system for use with a radio amplifier, and in which the amplifier output is demodulated, but is further amplified before application to the rectifier which delivers a direct current component for automatic control. An object is to provide an improved control system of the type employing a single tube as a demodulator and rectifier, and in which the tube performs each of these functions in a satisfactory and an efiicient manner.

A further object is to provide an automatic gain control system in which a multiple element tube and its circuit elements are so related that the tube functions as a power detector of the plate circuit rectification type, as an amplifier of the radio frequency, andas a diode for rectifying the amplified radio frequency energy to provide a direct current output proportional to the mean signal voltage. A further object is to provide an automatic gain control system of the type last stated, in which the single tube is a pentode of known type, or a tube of a special type, in which an anode element cooperates with the cathode and control grid to function both as a power detector and as an amplifier of radio to the anode of the diode may be of different values.

These and other objects and advantages of the invention will be apparent from the following specification, when taken with the accompanying drawing, in which- Fig. 1 is a circuit diagramof an embodiment of the invention in which use is made of a pentode tube of known type having control, screen and suppressor grids,

Fig. 2 is a circuit diagram of another embodiment in which is employed a tube resembling a pentode but having, in addition, a second anode or plate, and r Fig. 3 is a diagram illustrating another circuit arrangement of the general type shown in Fig. 1. a In the drawing, the reference numeral 1 identifies an amplifier of any desired type which amplifies radio energy received from a collector structure 2, at the received carrier and/or at an intermediate frequency, and delivers the amplified signal energy to thetuned input circuit L, C, of the combined demodulator and automatic gain control system. The gain of the amplifier is controlled, in the known manner, by an initial direct current bias on the control grids of some or all of the amplifier tubes and a second direct current grid bias obtained by rectification of the amplified radio'voltage. The detailed construction of the amplifier is not, however, an essential part of the present invention.

As shown inFig. l, the tube 3 which serves both as the detector and as the rectifier may take the form of a pentode of known type having a cathode K, a control grid G1, a screen grid G2, a suppressor grid G3 connected to the cathode K, and a plate P. The high potential terminal of the input circuit LC is connected to the control grid G1 and the low potential terminal is grounded, for radio frequencies, by a by-pass condenser '4, and is connected through a bias resistor 5 to a point which has a positive direct current potential with respect to ground, for example to a point on the resistor 6 which may be, and preferably is, the voltage divider or B- stick resistance of a power pack or rectifierfilter system for supplying a smooth direct current from an alternating current power line. The cathode K is grounded for signal frequencies by a condenser 1 and is conductively connected to a pointon' the voltage divider B which is more positive, by a direct current potential E1, than the voltage applied to the grid, thereby placing on the control grid of tube 3 a negative bias of E1 volts appropriate for plate circuit rectification in a power detector.

The screen grid G2 of tube 3 serves as the anode of the power detector and its audio frequency output circuit includes a radio frequency choke 8, an audio frequency load which is indicated generically at 9, and a source of direct current potential, such as the voltage divider resistor 6. A radio frequency by-pass condenser I is connected across the audio frequency load in the usual manner.

From a point between the screen grid G2 and the choke 8, a radio frequency circuit is connected between the grid G2 or detector anode and ground, and comprises a condenser II, the primary winding l2 of a transformer and a second condenser 13, the condensers ll, 13 having a low impedance for radio frequencies and a high impedance for audio or modulation frequencies. The secondary winding I4 of the transformer is connected to the plate P and, through a resistor 15, to ground. A condenser C1 is connected between plate P and ground for tuning the second-- ary winding Hi to the radio frequency, the tuned circuit including the condenser C1, inductance l4 and the by-pass condenser l3. When the amplifier I is of the tuned radio frequency type, the condensers C .and C1, preferably comprise similar sections of a gang condenser and, when the tube 3 is the second detector of a superheterodyne receiver, the condensers C and C1 are alining condensers which require no adjustment during the normal operation of the receiver.

The plate P cooperates with the cathode K to function as a diode rectifier having, in series between cathode .and anode, the resistor [5 across which a rectified direct current voltage is developed, and a tuned input circuit C1, l3, I4 upon which amplified radio energy is impressed by the primary winding l2 in the radio frequency circuit Il l2, l3 of the screen grid G2. The terminal 5 of the resistor I5 which joins the winding I4 is connected through an audio frequency .filter comprising resistor I7 and condenser l8, to the grid bias lead IQ of one or more of the radio amplifier tubes.

The conventional detector circuits which bypass the audio output impedance for signal frequencies are to be distinguished from the present invention in which the detector output includes, in parallel, the audio frequency or load impedance and an impedance resonant at signal frequency. The invention therefore provides a detector circuit which amplifies at carrier frequency in addition to its function of demodulating the received signal.

When a signal frequency voltage E0 is impressed upon the detector, an audio frequency voltage E9. is developed across the load impedance 9 and a radio frequency voltage Ep is developed between the plate P and ground by the fiow of amplified radio energy in the primary winding l2. The amplification may be of such order as to make Ep about three times as great as Eu. The amplified signal frequency voltage Ep is rectified by the diode action between plate P and cathode K to produce a direct current voltage E3 across the resistor l5. The polarity of this voltage E3 is such that terminal iii of the resistance is negative with respect to ground, and the voltage E3 increases as the radio voltage E0 increases.

The phase of the voltage Ep on the plate P is opposite to that of the radio frequency voltage on the screen grid G2 since, in general, a larger variation in the direct current voltage E3 will obtain with this phase reversal. The reversal of phase is most readily obtained by tapping the terminal l6 of resistance I5 to an intermediate point on a continuous winding, thereby dividing the winding into the two transformer sections 12, I4. 5

Particular attention is directed to the fact that the bias voltage E1 on the control grid G1 has, in general, a value different from that of the bias voltage E2 on the anode element P of the diode rectifier. An independent variation of the two bias voltages permits the proper adjustments for best operating conditions when a single tube serves as the demodulator and also as the rectifier element of an automatic gain control system.

The negative bias voltage E2 on the plate P effects a delay in the action of the automatic gain control since there will be no rectification by the diode until the peak value of the amplified radio voltage E1) is greater than the negative bias E2. This is essential for good control as it permits maximum amplification for all weak signals and a rapid development of the automatic gain control voltage E3 as the received radio energyincreases above the critical value which, with maximum amplifier gain, develops an input voltage E0 sufilcient to provide a satisfactorily high audio level with weak modulation. Overloading of the detector with a resultant distortion in the audio output will result when the detector input voltage E0 exceeds a definite value which is determined by the control grid bias.

According to the invention, the detector input voltage may be safely increased up to approximately the critical overload voltage since the gain control voltage is not obtained by rectification of the detector input voltage E0 but by the rectification of the amplified radio frequency voltage Ep.

In general, the control grid bias E1 and the diode bias E2 should be 50 related that the development of a rectified voltage E3 begins when the detector input voltage E0 rises to that level which permits overloading of the succeeding audio load circuit 9 by a weakly modulated carrier input when the amplifier I is operated at maximum gain. The development of a gain control voltage by rectification of a radio voltage Ep of greater magnitude than the detector input voltage E0 results in a relatively large gain control voltage E3 which varies rapidly with changes in the detector input voltage E0 and which, at a radio voltage E0 high enough to overload the detector, will be larger than the negative bias potential required to reduce the amplifier gain for the reception of the maximum radio energy obtainable at the collector structure 2.

The following data are given as indicative of values which resulted in satisfactory operation when the tube 3 was the second detector of a superheterodyne receiver having an intermediate frequency amplifier tuned to 175 kilocycles:

Condenser 4:0.1 microfarad Condenser 1:0.5 microfarad Condenser 01:50-100 micromicrofarads The described tube construction and circuit ar- 75 rangements are not essential to the-practice of the invention as tubesof other types may be employed in place of the pentode 3 or combinations of two or more tubes maybe substituted for the single tube.

'As shown in Fig. 2, the tube 3 is of the type described in U. S. Patent 1,954,195 of Knox C. Black, issued April 10, 1934 on, application Ser; No. 446,098, filed April'21, 1930, and differs from the pentode 3, as described above, by the addition of a second plate P. The tube 3,functions in the usual manner as a pentode detector with a control grid G1, a screen grid G2, suppressor grid G3 and plate P. The second plate P cooperates with the cathode K to form the diode for the rectification of the amplified radio voltage which is developed by the amplification action of the pentode elements. As distinguished from the Fig. 1 circuit, the positive direct current potential impressed upon grid G2 causes this grid to function as-the usual screen grid, theprimary winding I2 is connected to the plate P through the condenser H, and the outer terminal of the secondary winding I4 is connected to the second plate P. With these exceptions, the circuit elements and their relationships are or may be the same as that shown in Fig. 1. The several circuit elements will not be described in detail but are identified by the reference numerals ap-' plied to corresponding elements in Fig. 1. It will be apparent that other circuit elements may be employed for transferring the amplified voltage of radio frequency from the demodulator output circuit to the rectifier. In the circuit shown in Fig. 3, the general construction of the tube inputand audio output circuits is substantially identical with that shown in Fig. 1 and corresponding elements are therefore identified by the same reference numerals. The radio fre quency load in the demodulator output circuit comprises the serially arranged inductance l2 and adjustable condenser C1 which are connected between the screen grid G2 and the cathode through the condensers l! and I. The junction of the inductance l2 and condenser C1 is connected to the plate P, thus applying the voltage drop across the condenser C1 to the diode plate and thereby effecting a phase reversal between the radio frequency voltages onthe screen and plate of tube 3. The rectified radio voltage is developed across the resistor 15 which is connected between ground and the screen terminal of the inductance l2.

A condenser 20 connected'between the cathode K and the screen grid G2 has a relatively high impedance for radio frequencies but a comparatively low impedance for voltages of substantially higher frequencies. The impedance of the output circuit of the demodulator for frequencies above the radio frequency is determined principally by the capacitive reactance 20 and decreases with increasing frequency, thus suppressing the development, in the output circuit, of those impressed voltages which are harmonics of the signal fre quency. In this respect, the Fig. 3 circuit is distinguished from the Figs. 1 and 2 circuits in which the impedance of the demodulator output circuit for frequencies above the radio frequency is determinedby the reactance of the inductance I2, and therefore rises with increasing frequency.

The use of an output load which is capacitive at frequencies higher than the radio frequency 'is particularly advantageous when the tube 3 is the second detector of a superheterodyne receiver. H rm n c 9f. he n rm d e u pcy v l e impressed on tube are disturbing when fed back to carrier frequency circuits preceding the first detector since they may combine, at certain oscillation frequencies, with an undesired carrier frequency to produce a beat voltage of the desired intermediate frequency.

When working out of an intermediate frequency amplifier that is tuned to kilocycles, the condenser 20 may have a value of about 1000 micromicrofarads, the inductance l2 being about 9.5 millihenries, and the remaining elements having the values as previously noted in connection with the Figs. 1 and 2 circuits.

It will be apparent that the circuit shown in Fig. 3 may be adapted for use with a special tube of the type shown in Fig. 2.

The described embodiments of the invention are particularly advantageous in that a tube may be operated up to its full distortionless output as a.

power detector, and the same tube also serves as the source of direct current potential for an automatic gain control system which becomes effective only when the received radio energy rises above the value which, at maximum amplification, will provide a normal detector input voltage.

I claim:

1. In an automatic gain control system, the combination with a tube having a cathode, a control grid and a pair of anodes; and means establishing between said tube electrodes energizing potentials effective to cause the cathode and one anode to function as a diode rectifier, and to cause the cathode to cooperate with'the grid and the second anode to constitute a detector, an input circuit for impressing a radio voltage between grid and cathode, an output circuit between the second anode and cathode, said output circuit including a circuit element across which an amplified radio voltage is developed, a rectifier output circuit including a resistance, and means impressing uponthe first anode a radio voltage proportional to the said amplified radio voltage and of a phase opposite that of the radio voltage between said second anode and cathode.

2. In a radio receiver, the combination with a radio frequency amplifier, of a multiple function stage for developing a direct current and an audio frequency voltage, said stage comprising a single tube having a detector anode and a rectifier anode each cooperating with a cathode, a detector input circuit coupling said tube to the radio frequency amplifier, an audio frequency output circuit connected between said detector anode and its cooperating cathode, a rectifier input circuit impressing on said rectifier anode a radio frequency voltage of a phase substantially opposite to that of the radio frequency voltage on the detector anode, an output resistance across which a direct current voltage is developed by said rectifier, and control means energized by the said direct current voltage.

3. In an automatic gain control system, the

pedance for radio frequency voltages, means impressing between the first anode and the cathode the amplified radio voltage developed across said circuit element, and a resistance in the output circuit of said first anode.

4. In an automatic control system, the combination with a detector tube having a cathode cooperating with a control grid and an output electrode, a radio input circuit for said tube, and an output circuit across which an audio frequency and an amplified radio frequency potential are developed, of a rectifier comprising the cathode of said tube and an anode located outside of the electron stream controlled by said grid, means impressing on said rectifier the amplified radio frequency potential developed in the said output circuit, and an output resistance across which a direct current potential is developed by said rectifier, said resistance constituting a source of direct current voltage for application to a control circuit.

5. In a radio receiver, the combination with a radio amplifier, of a combined detector-automatic gain control stage working out of said amplifier, said stage including a single tube housing the electrodes of a diode rectifier and a pentode, a radio frequency input circuit for said pentode, an output circuit for said pentode including an impedance across which an amplified radio frequency voltage is developed, means impressing the said amplified voltage on said diode rectifier, a resistance across which said rectifier develops a direct current voltage, and circuit elements for returning said direct current voltage to said radio amplifier to control the gain thereof.

6. In a radio receiver, the combination with a radio amplifier, and a single multiple function tube working out of said amplifier; said tube having therein a cathode and the cooperating electrodes of a pentode amplifier, and an anode located outside of the electron stream of said amplifier and cooperating with said cathode to constitute a diode; of a tube input circuit coupled to said radio amplifier, an output network for said tube; said network including impedances across which are established an amplified radio frequency potential and an audio frequency potential, a re sistance in the output circuit of said diode and included in a biasing circuit of said radio amplifier, and means impressing on said diode the amplified radio voltage developed in said output network by said tube, whereby the bias voltage returned to said radio amplifier is of greater magnitude than that obtainable by diode rectification of the radio voltage across said tube input circuit.

7. In a radio receiver, the combination with a radio frequency amplifier, and a single tube having a cathode and a plurality of electrodes cooperating therewith, said electrodes including a control grid, of input circuit connections for impressing between said control grid and cathode the amplified radio output of said radio amplifier, means including circuit elements cooperating with said cathode and certain of said electrodes to constitute an output network in which an amplified radio voltage and an audio frequency voltage are developed, an additional electrode in said tube and located outside of the electron stream in which said control grid is located, means including circuit connections for impressing between said additional electrode and cathode the amplified radio voltage developed in said output network, a resistance connected between said additional electrode and cathode, said additional electrode and cathode constituting a diode rectifier developing a direct current potential across said resistance, and circuit connections for returning the direct current potential to the radio amplifier as a gain control bias.

8. A multiple-function stage adapted to work out of a radio amplifier to develop an audio frequency voltage for application to a load circuit and a direct current voltage for return to the amplifier as a gain control bias; said stage comprising a single tube housing a cathode and the cooperating electrodes of an amplifier, and an anode located outside the main electron stream of said amplifier and forming one element of a diode; a radio frequency input circuit for said tube, an output network for said tube and including impedances across which an amplified radio voltage and an audio frequency voltage are developed; means impressing the amplified radio voltage between said anode and cathode, and an output circuit between said anode and cathode, said output circuit including a resistance across which a direct current voltage is developed.

9. In an automatic gain control system, the combination with a tube having a cathode, a control grid and two anodes; and means establishing on said tube electrodes direct current potentials eifective to cause the cathode and one anode to function as a diode rectifier and to cause the cathode to cooperate with the grid and the other anode to constitute a demodulator; of an input circuit connected between grid and cathode and resonant at a radio frequency, an output circuit between said second anode and said cathode, said output circuit including impedances across which the demodulator elements establish an audio frequency potential and an amplified radio frequency potential, a rectifier circuit between the cathode and said first anode, said rectifier circuit including a radio frequency input element and a resistor, and means coupling said output circuit to said rectifier input element to impress upon the rectifier anode a radio potential of opposite phase to the radio potential of the demodulator anode.

10. In a receiver, the combination with a radio frequency amplifier, of a combined demodulator and automatic gain control system for said amplifier, said combined system including a pentode tube of the type having suppressor and screen grids, an input circuit for said tube and coupled to the amplifier, an output circuit between the screen grid and cathode providing a radio frequency and an audio frequency load, the radio frequency load comprising the primary of a transformer tuned to the radio frequency, a circuit including a resistor and the secondary of said transformer connected between the cathode and plate of said tube, and means for transferring the rectified voltage developed across the said resistor back to said amplifier as a gain control voltage.

11. The invention as set forth in claim 10, in combination with means for applying to the plate a direct current potential which is negative with respect to the tube cathode.

12. The invention as set forth in claim 10, in combination with means for individually adjusting the direct current potentials, with respect to the cathode, of the control grid and plate of said tube.

13. In an automatic gain control system, the combination with a radio frequency amplifier, a tube having a cathode, a control grid, a screen lator elements establish an audio frequency potential and an amplified radio frequency potential; means transferring said amplified radio frequency potential to the diode rectifier circuit comprising the cathode and other anode, a resistor across which a rectified radio voltage is developed by said diode rectifier, and means transferringthe rectified voltage developed across the said resistor back to said radio frequency amplifier as a gain'control voltage. 7

14. The invention as set forth in claim '13, in combination with means for applying to the diode rectifier anode adirect current potential which is negative with respect to the tube cathode.

15. The invention as set forth in claim 13, in combination with means for individually adjusting the direct current potentials, with respect to the cathode, of the control grid and diode rectifier anode of'said tube.

16. In a radioreceiver, a radio frequency amplifier including a final radio frequency stage working into a succeeding stage having an audio frequency impedance between the input terminals thereof/said final stage comprising a single tube housing the elements of a radio frequency amplifier andthe elements of a diode rectifier, said rectifier elements being located outside the main electron stream of said amplifier, an input circuit connected between the amplifier input elements of the tube, an output network for said amplifier elements and including terminals across which said succeeding stage may be coupled, means impressing across the diode elements the amplified radio frequency voltage developed in said output network, a resistance acrosswhich a direct current voltage is developed by rectifier action of said diode, and means returning to the radio frequency amplifier a gain control bias proportional to said direct current voltage.

1'7. In a multistage transmission system, a multifunction stage comprising a vacuum tube having an auxiliary anode additional to the tube elements normally required for amplification, said auxiliary anode cooperating with the tube cathode to constitute a diode rectifier, input and output circuit elements cooperating with said tube elements to form a radio frequency amplifier, an input circuit for said diode coupled to one of said output circuit elements, an output impedance for said rectifier and across which a direct current voltage is developed by the rectifier action, and a gain control circuit for automatically varying the gain of said transmission system to maintain a substantially constant voltage level at a predetermined point thereof, said gain control circuit including the said rectifier output impedance.

18. In an amplifier, a demodulating device having a signal output circuit, and means for deriving a biasing potential from current flowing in said circuit, said means including an anode electrode in said device and a device for coupling said anode electrode to the output circuit.

19. In an amplifier, a demodulating device having a cathode and a plurality of output electrodes, 4

an output circuit connected to one of said output electrodes across which circuit, when the amplifier is in operation, appear potentials representative of the modulation of an incoming modulated carrier as well as potentials representative of the carrier per se, and means for re-"impressing the potentials representative of the carrier between another of said output electrodes and. said cathode to thereby provide unidirectional current proportional to the amplitude of the carrier.

20. In an automatic gain control system, the combination with a tube having a cathode, acontrol grid and two anodes; and means establishing on said tube electrodes positive potentials effective to cause the cathode and one anode to function as a diode rectifier and to cause the cathode to cooperate with the grid and the other anode to constitute a demodulator; of an input circuit connected between grid and cathode and resonant at a radio frequency, an output circuit between said second anode and said cathode, said output circuit including impedances across which the demodulator elements establish an audio frequency potential and an amplified radio frequency potential, a rectifier circuitbetween the cathode and said first anode, said rectifier circuit including a radio frequency input element and a resistor, andmeansfor applying the said amplified radio frequency potential to said rectifier input element, thereby to produce a rectified radio voltage across said resistor which is of greater magnitude than the voltage obtainable by diode rectification ofthe radio voltage existing'across said input circuit."- I I a I 21. The combination with a radio frequency amplifier, of a detector-amplifier working out of said'radio frequency amplifier, said detector-amplifier comprising a vacuum tube having a cathode and a plurality of electrodes, means energizing two of said electrodes to cooperate with said cathode as the control grid and anode of a detector, an audio frequency load in the output of said detector, and means having a capacitive reactance at frequencies substantially above the radio frequency range of said amplifier constituting a radio frequency load in the output circult of said detector.

22. In a signal receiving system, the combination with a signal input circuit, of a rectifier means connected therewith and including an anode, a cathode, and a potential drop-producing means interposed therebetween in circuit with said input circuit, and an amplifier means including a control electrode and an anode associated with said cathode, and a second signal input circuit connected with said control electrode and with said potential drop-producing means, whereby said control electrode may receive a biasing potential from said last-named means.

23. In a radio receiver, the combination with a radio signal amplifier of an electric discharge device having a cathode, a rectifier anode, a control grid, and a positive output anode, a signal input circuit for said device connected between the cathode and the rectifier anode and including in series tuning means and a resistor, a signal bypass condenser connected between said resistor and cathode, an audio frequency output circuit connected between the cathode and the positive output anode, means for deriving an amplifier control potential from said resistor, and means for impressing between said control grid and cathode a biasing potential effective to secure amplification of audio frequency variations impressed upon said control grid.

24. In a radio signal amplifier, the combination of an electric discharge device for demodulating a signal, said device having at least two output electrodes and output circuits connected therewith, means for deriving currents from said demodulating device at audio frequencies, means providing output current from said device for biasing purposes proportional to the amplitude of said signal, and means for interconnecting said output circuits whereby they are arranged in energy inter-change relation to each other.

25. The combination with a radio frequency amplifier, of a detector-amplifier working out of said radio frequency amplifier, said detectoramplifier comprising an electronic tube having a cathode and a plurality of electrodes, means energizing two of said electrodes to cooperate with said cathode as the control grid and anode of a detector, an audio frequency load means in the output of said detector, and impedance means resonant at frequencies in the radio frequency range of said amplifier constituting a radio frequency load in the output circuit of said detector.

26. In a transmission system, a radio frequency amplifier including a final stage comprising an electron-discharge tube having a radio frequency and an audio frequency output circuit, a diode rectifier connected across said radio frequency output circuit, and means returning to said amplifier as a gain control voltage the direct current potential developed by said diode rectifier, said diode rectifier comprising elements housed within said electron discharge tube.

27. In a transmission system, a radio frequency amplifier including a final stage comprising an electron-discharge tube having a radio frequency and an audio frequency output circuit, a diode rectifier connected across said radio frequency output circuit, means returning to said amplifier as a gain control voltage the direct current potential developed by said diode rectifier, said diode rectifier comprising elements housed within said electron discharge tube, and means impressing on said diode rectifier a bias voltage which renders the same inoperative when the radio output voltage of said amplifier falls below a predetermined critical value.

28. In a transmission system, a multiple function stage comprising a tube housing a cathode cooperating with a control grid and plate to form an amplifier and a diode rectifier anode, a radio input circuit connected between said control grid and cathode, an output circuit in which an amplified radio voltage is developed by the action of said amplifier elements, means impressing said amplified voltage between said anode and cathode, a resistance across which a direct current voltage is developed by the diode rectifier action of said anode and cathode, a signal amplifier, and means impressing on said amplifier at least a part of the voltage developed across said resistor, wherein said output circuit includes a cathode bias resistor across which plate current fiow develops a direct current potential that makes the cathode positive with respect to the rectifier anode.

29. In an amplifier, an electron discharge device having a signal output circuit, means for deriving a biasing potential from current flowing in-said circuit, said means including an anode electrode in said device, and a device for coupling said anode electrode to the output circuit.

PAUL O. FARNHAM. 

